Methicillin-resitant Staphylococcus aureus (MRSA) are the most common and lethal causes of infections of hospitalized patients and, over the past decade, have appeared in the community as an increasingly common cause of infections of individuals who both have and do not have healthcare exposures. MRSA are resistant to beta-lactam (penicillin and derivatives) and many additional antibiotics (multiresistant), severely limiting options for treating serious infections. Beta-lactam antibiotics target the enzymes (penicillin binding proteins or PBPs) that maintain the integritiy of the bacterial cell wall. The gene responsible for methicillin resistance (MR), mecA, encodes a new PBP (PBP2a) that can maintain cell wall structure but is resistant to beta-lactam inhibition mecA is carried on a genetic element called a genomic island that inserts into the staphylococcal chromosome at a specific sequence (attB). In addition to housing mecA, this island, called SCCmec, carries genes called ccr that catalyze both insertion and excision of SCCmec. Both the mechanisms of insertion and excision and the epidemiology of various forms fo SCCmec suggest that the element is mobile and has moved among staphylococcal strains multiple times in the past thirty years. However, the genetic types of SA that have acquired SCCmec are limited compared to the wide range of strain types of methicillin susceptible (MS) SA that are available. This proposal seeks to understand how SCCmec is transferred among staphylococcie and the specific genetic requirements of recipient MSSA. In addition, it will investigate how SCCmec can be lost, converting MRSA back to MSSA. The tree Specific Aims are to 1) Elucidate the molecular mechanisms and target sequences required for ccr-mediated SCCmec insertion and excision; 2) Assess the frequency of spontaneous SCCmec excision in vitro and in vivo; and 3) Capture excised SCCmec on a conjugative plasmid, transfer SCCmec and mecAto suitable staphylococcal recipients and identify genomic changes associated with acquisition and stable maintenance. In addition, since there is epidemiologic and genomic evidence that S.epidermidis (SE), a less virulent staphylococcal species, also carries SCCmec and can serve as a genetic reservoir for this element, we will perform experiments in both SA and SE. The goal of these studies is to identify sequences in SA and SE that can be used as probes to dissect the epidemiology of the spread of MR. In this manner we can trace the history of the emergence and rapid spread of MRSA, prevent future dissemination of SCCmec and support arguments that reduce antibiotic will favor SCCmec loss from host bacteria and lower the environmental prevalence of the MRSA phenotype.